1. Field of the Invention
The present invention relates to a phase control apparatus to demodulate a signal light beam that is differential-quadrature-phase-shift-keying (DQPSK)-modulated and an optical DQPSK receiver.
2. Description of the Related Art
Conventionally, a receiver having the configuration as shown in FIG. 16 is known as an optical DQPSK receiver 10 that receives a signal light beam that is differential-quadrature-phase-shift-keying-modulated (for example, International Patent Application Publication No. 2004-516743). In the optical receiver 10, a DQPSK signal input is branched into two. The branched light beams are provided respectively to delay interferometers 11 and 12. Each of the delay interferometers 11 and 12 is adapted to differ the optical path lengths of two optical waveguides from each other that are Mach-Zehnder-type optical waveguides formed on, for example, a silica substrate or an indium phosphide substrate, thereby generating a relative delay time difference corresponding to at least an integral multiple of one symbol that is DQPSK-modulated between light beams propagating the optical waveguides.
The interference operating point of the delay interferometer 11 is defined to be π/4 by a delaying unit 13 formed on one of the optical waveguides. The interference operating point of the delay interferometer 12 is defined to be −π/4 by a delaying unit 14 formed on another one of the optical waveguides. Complementary two outputs that are output from a coupler at the output-stage of the delay interferometer 11 are received by a differential optical receiving circuit 15 including a pair of optical detectors and an amplifier. The differential optical receiving circuit 15 demodulates an electric signal A corresponding to a signal D1 input into a transmitter not shown.
Similarly to the above, complementary two outputs that are output from a coupler at the output-stage of the delay interferometer 12 are also received by a differential optical receiving circuit 16 including a pair of optical detectors and an amplifier. The differential optical receiving circuit 16 demodulates an electric signal B corresponding to a signal D2 input into a transmitter not shown. The electric signals A and B that have been demodulated are newly created as more stable electric signals respectively by clock data recovery (CDR) circuits 17 and 18. Synchronization process of frames and creation of frames such as synchronous digital hierarchy (SDH), synchronous optical network (SONET), or oracle technology network (OTN) and error correction are executed by a framer circuit/forward error correction (FEC) decoding circuit 19.
For such an optical DQPSK receiver, it is important that optical phase differences between the optical waveguides of the delay interferometers 11 and 12 are respectively set accurately to be π/4 and −π/4. Such an optical DQPSK receiver is known that a heater is provided to each of one optical waveguide of one delay interferometer and one optical waveguide of the other delay interferometer, to adjust independently the temperature of each of the heaters, thereby adjusting the optical phase difference between the optical waveguides of the delay interferometers 11 and 12 (for example, International Patent Application Laid-Open Publication No. 03/063515).
When an optical DQPSK receiver is actually used, at the time of, for example, starting up of the receiver, switching of optical signals, or fluctuation of external conditions such as the environmental temperature, the optical phase difference between the optical waveguides of each of the delay interferometers needs to be adjusted to a desired state in a short time. However, in International Patent Application Publication No. 2004-516743, it is not disclosed that how the optical phase difference between the optical waveguides of each of the delay interferometers is controlled. In International Patent Application Laid-Open Publication No. 03/063515, it is not disclosed that how the temperature of each heater is controlled.
Therefore, in the above conventional receivers, the adjustment of the optical phase difference can not be completed in a short time because the optical phase difference and the temperature of each of the heaters are adjusted in a trial-and-error manner. That is, the optical receivers disclosed in the above patent literatures are still in development stages and are not suitable for practical use. The optical receiver disclosed in International Patent Application Laid-Open Publication No. 03/063515 has a problem that the optical phase difference between the optical waveguides can not be fully controlled because the range of temperature adjustment by a heater is narrow.